Aqueous ink compositions involved various defects. That is, in the case where the aqueous ink composition is printed on plain paper, it is inferior in water resistance and is liable to cause blotting (i.e., blurring). Further, in the case where it is printed on a non-water absorbing material to be recorded, such as plastics, since attachment (i.e., adhesion) of ink droplets is poor, an image cannot be formed; since drying of the solvent as used is extremely slow, it is required to dry printed materials immediately after printing without being piled up; and the resulting image is liable to cause blotting.
In order to solve these problems, EP997508A1 discloses inkjet inks that are cured with radiations and fixed. However, since the disclosed inkjet inks use a pigment dispersion as a coloring agent, the transparency is poor, and the color tone is not satisfactory, and hence, it is difficult to obtain photographic images. Further, there was involved a problem such that nozzles are clogged by coagulation of the pigment so that it is difficult to stably eject the ink.
As means for solving this problem, it may be considered to employ a method of using a dye as the coloring agent. However, the dye very likely worsens the radical polymerization properties, and hence, it was difficult to use it for curable inks. U.S. Pat. No. 4,303,924 discloses inks in which a dye not retarding the curing is used. However, this U.S. patent does not definitely describe a guideline of selection of an effective dye. Moreover, according to the conditions illustrated in the U.S. patent, the dye density is low, and the curing conditions under nitrogen, which are free from polymerization retardation by oxygen, are used. In the case where the dye density is increased, or the curing is carried out in air, the curing becomes extremely difficult. Accordingly, for the practical use, there is a large restriction in facilities for providing the conditions under nitrogen.
In addition, the inks as disclosed in the U.S. patent contain conductive salts, and in many cases, these conductive salts are poor in solubility in the ink. Accordingly, there was a possibility that these conductive salts deposit during the storage over a long period of time, resulting in poor printing.